1. Field of the Invention
This invention relates to a muffler (silencer) used in an internal combustion engine for an automobile and the like.
2. Related Art
There is known a muffler in which when an internal combustion engine is operating at high (engine) revolution speed at a high flow rate of exhaust gas, the reduction of a back pressure is regarded as important so as to enhance an output power, and when the engine is at low engine revolution speed at a low flow rate of the exhaust gas, a sound-suppressing effect is regarded as more important than the reduction of the back pressure is. Such a muffler as shown in FIG. 4 is disclosed in JP-Y2-02-18265.
The muffler, shown in FIG. 4, comprises a shell 100 whose interior is partitioned into a first chamber 101, a second chamber 102 and a third chamber 103, an exhaust inlet pipe 104 having perforations 105 through which the interior of this pipe 104 communicates with the first chamber 101, and having one end open to the second chamber 102, a communication pipe 107 which has perforations 106 through which the interior of this pipe 107 communicates with the second chamber 102, and has opposite ends open respectively to the first chamber 101 and the third chamber 103, a first exhaust outlet pipe 109 which has perforations 108 through which the interior of this pipe 109 communicates with the third chamber 103, and has an inner end open to the first chamber 101, and also has an outer end open to the exterior of the shell 100, a outlet pipe 110 connected to the outer end of the first exhaust outlet pipe 109, and a throttle valve 111 provided between the first exhaust outlet pipe 109 and the tail pipe 110.
The muffler further comprises a second exhaust upstream pipe 112 which extends through the first chamber 101 and the second chamber 102, and has an exhaust inlet end open to the third chamber 103, and a second exhaust downstream pipe 113 of a generally U-shape connected to a downstream end of the upstream pipe 112 to form, together with this pipe 112, a second exhaust outlet pipe 114. The second exhaust downstream pipe 113 is turned or bent into a U-shape, and extends along an outer surface of a side wall of the shell 100, and a downstream end of this pipe 113 is connected or joined to that portion of the tail pipe 110 disposed downstream of the throttle valve 111.
The throttle valve 111 comprises an electromagnetic valve (solenoid valve), and the engine revolution speed is detected, and when the engine revolution speed is high, the throttle valve 111 is opened under the control of a control circuit 115, and when the engine revolution speed is low, the throttle valve 111 is closed.
When the engine revolution speed is high, the throttle valve 11 is opened, thereby discharging the exhaust gas, flowed into the muffler through the exhaust inlet pipe 104, through the first exhaust outlet pipe 109 so as to reduce a back pressure. When the engine revolution speed is low, the throttle valve 11 is closed, thereby discharging the exhaust gas, flowed into the muffler through the exhaust inlet pipe 104, through the second exhaust outlet pipe 114 (which is smaller in diameter, and is longer than the first exhaust outlet pipe 109) so as to enhance a sound-suppressing effect.
In the above conventional muffler, however, the second exhaust upstream pipe 112 extends through the two chambers, and the second exhaust downstream pipe 113 extends from a front wall of the muffler in a U-turn manner, and further extends along the side wall of the shell toward a rear end thereof. Therefore, the overall length of the second exhaust outlet pipe 114, constituted by the two pipes 112 and 113, is about twice larger than the overall length of the muffler. The second exhaust outlet pipe 114, having such a large length, has a relatively low columnar resonance frequency.
With respect to the pulsation of the engine, the lower the engine revolution speed is (that is, the lower the frequency is), the stronger the pulsating effect is.
Therefore, in a low engine revolution speed range, the frequency of such intense engine pulsation and the columnar resonance frequency of the second exhaust outlet pipe coincide with each other, and this results in a problem that an intense columnar resonance occurs as indicated by a peak C' of a broken-line curve C (FIG. 5) representing a sound pressure level of the conventional muffler, so that abnormal sounds are produced.
Furthermore, the second exhaust outlet pipe 114 of the above construction is arranged to be projected at the rear side and one side of the muffler, and therefore there is encountered a problem that the muffler can not be formed into a compact design.